Offset lithographic printing is a method employing a plate cylinder which carries a planographic printing plate, whose image and non-image portions are substantially coplanar, the image area being hydrophobic and the non-image area being hydrophilic. A dampening system applies aqueous solution to the non-image areas of the plate, and an inking system applies a greasy ink to the image portions. The thus coated plate is rotated into contact with a resiliently surfaced blanket cylinder, thereby transferring the ink (and dampening solution) from the plate cylinder to the blanket cylinder. Printing stock, in either sheet or web form, is fed against the blanket cylinder by an impression cylinder, and the ink (and dampening solution) is transferred to the stock, thus completing the printing operation except for any heat-drying which may be employed.
Flexographic printing represents a different approach than offset printing, and its most common applications involve printing on web stock employed in packaging, such as corrugated stock, plastic, etc. Printing plates are employed in which the image areas are raised with respect to the non-image areas, rather than being co-planar with them. There is no blanket cylinder, and the plate cylinder comes into direct contact with the stock, which is delivered against it by an impression cylinder. Ink is applied to the image areas of the plate by rollers working in an ink pan adjacent the plate cylinder.
Each of these two methods of printing has its own set of advantages and disadvantages, but as the art has developed thus far, it has not been possible to operate both methods on a single press, using whichever is most advantageous for a given job. In the art of offset printing, techniques of sheet feeding are highly developed, while, as pointed out above, web feed is normative for flexographic presses, and the advantages of flexographic printing are largely foregone in sheet-fed applications.
In the field of offset printing, development work is being done on catalyst-set inks and other two-part ink systems. One difficulty with catalytic systems is the tendency of the catalyst to back-migrate from its point of application to the plate cylinder into the ink fountain, where it causes premature setting of the ink.
In some printing applications, it is desirable to coat the stock with a varnish or other coating after the printing has been applied, instead of printing on top of stock precoated by the paper manufacturer. For such applications, printing houses have a need for equipment capable of applying a suitable over-coating to printed stock. Preferably, such equipment should be capable of applying the coating simultaneously with the printing, but in any event, it should be capable of applying the coating in a separate step.